Make sure you don't try to reuse the old HD as your blade SSD in the system will try to recover the Fusion Drive set with your HD is it sees it. You should have broken the drive set before removing the old drive.

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I would think long and hard here before attempting this. First these systems are a bear to open! Many people have damaged their displays in the process in these newer 'Thin Series' iMacs. If you do attempt this make sure you have the right tools and have a few sets of adhesive strips (in case you need to reopen).

OK, lets hit the different things you have here:

You want to upgrade the Apple SSD blade drive.

Larger SSD's are not presently available from Apple and so far no 3rd parties that offer upgrades are offering SSD upgrades for the newer 'Thin Series' iMac's. I should also point out you need to completely remove the logic board to even gain access to the SSD socket. Here's the IFIXIT guide as a reference: iMac Intel 27" Retina 5K Display SSD Replacement.

You want to swap out the HD of your Fusion Drive out for a SSD.

Before you do this you need to break the Fusion Drive set first as if you replace the HD the blade SSD it was mated to will re-establish a Fusion set with your new drive (which you don't want to do here). You also want to make an external bootable USB thumb drive with the OS installer so you can reformat and install a fresh OS on your SSD.

To be honest here. You may want to think about selling your iMac and jumping up to a Mac Pro system if you want this level of power and more! It's a lot more serviceable and expandable both internally and externally.

I don't think that iMac 2013 later requires a thermal sensor cable if it is not using Seagate HDD because other brand HDDs are using S.M.A.R.T to communicate heat info. If you want to use SSD to replace HDD, the thermal sensor cable does not even exist in iMac 2013 and later. OWC is absolutely scamming.

@sheenbling - Sorry Evan, you do need it. It's not that simple. Apple is able to access S.M.A.R.T. services of the drive (HD or SSD). But thermal reporting within S.M.A.R.T. requires a hardware IRQ so it is not very timely. This is were Apple contracted the drive makers to make available the diagnostic sensor they had within the drive for burn-in during manufacturing. So, if you get a real Apple OEM drive you could do a swap-out without needing the sensor. But if you get a standard off the shelf drive you'll need the OWC in-line sensor for either HD or SSD.

Yes Dan, what you said is true for the system like Yosemite and older. If you are running El Capitan and Sierra on iMac 2012 later, you don't need the OWC cable because El Capitan's fan control works without checking the HDD temp. If you don't think so, you could install an SSD like Samsung 850 EVO on iMac 2012 and later running Sierra. The cooling fan should work as usual.

@sheenbling - SMC services is hardware based not OS based. You need to either off-set the lack of the sensor or you need to fake out SMC via a software over-ride. I recommend the hardware solution as over time we have learned it is the better solution.

I have a system which needs a new drive I'll double check to see if Apple altered things.

Cool. I understand that SMC is hardware based, however, the fan won't go crazy once you enter El Capitan or Sierra probably because the OS will not need the thermal sensor to collect the temp info in this case. Please try this on your iMac if it is possible. Thank you so much for discussing it with me. Let me know the result on your iMac. Very appreciate for make things clear.